Zaviyalov О. I., and Polivanov М. K. 1992. “R-Operation of Bogolyubov—Parasyuk and Operator Expansions in Mass-Free Theories ”.
Ukrains’kyi Matematychnyi Zhurnal
44 (3): 333-40.
https://umj.imath.kiev.ua/index.php/umj/article/view/8102
.